Information storage devices are manufactured in high volume and widely used to store and/or retrieve data in computers and other consumer electronics devices. Information storage devices may be classified as volatile or non-volatile, depending upon whether their memory content is maintained when the information storage device is not powered. Examples of non-volatile information storage devices include magnetic hard disk drives and magnetic random access memory (MRAM) devices, either of which may utilize a magnetoresistive tunnel junction (MTJ) as part of information storage or retrieval operations. Specifically, whereas volatile random access memory (RAM) devices typically store data as electric charge, MRAM devices may store data in MTJs that maintain memory content even when the memory device is not powered.
Generally, each MTJ includes a reference layer that has a magnetic orientation that is pinned or fixed, and a free layer having a magnetic orientation that can be changed by an external magnetic field (e.g. from an adjacent disk or generated by a programming current). The MTJ is in a low resistance state when the free layer magnetic orientation is parallel to that of the reference layer, and in a high resistance state when the free layer magnetic orientation is anti-parallel to that of the reference layer. If the external magnetic field and/or programming current required to switch a desired MTJ between high and low resistance states (with acceptable switching speed) is too great, or if the MTJs are arranged too closely together, then one or more adjacent MTJs may undesirably be affected or inadvertently switched.
There have been many patented variations and improvements to MTJs in recent years, some of which help mitigate the foregoing problem to allow for more reliable operation when the MTJs are arranged in close proximity to each other. For example, a spin transfer torque magnetic random access memory (STT-MRAM) has been investigated, in which each MTJ is switched (i.e. “programmed”) by an application of spin polarized current through the MTJ. The STT-MRAM promises significant advantages over magnetic-field-switched MRAM, which has been recently commercialized. For example, STT-MRAM can be scaled to a smaller size while maintaining the programmability of individual MTJs (without inadvertently and undesirably affecting the programming of adjacent MTJs). Moreover, STT-MRAM can be programmed with lesser programming currents, which reduces power consumption and associated requirements for heat dissipation.
However, one of the challenges for implementing STT-MRAM is minimizing the programming current required to quickly switch the magnetic orientation of the free layer in the MTJ, while maintaining high thermal stability of the recorded data. Reduced programming current may enable the use of smaller memory cell transistors, which can substantially reduce the overall size of the incorporating memory device. A reduced programming current requirement also corresponds to reduced voltages across the MTJs during writing and sensing, which may decrease the probability of tunneling barrier degradation and breakdown, and thereby improve the endurance and reliability of the incorporating memory device.
Hence, there is an ongoing need in the art for an improved MTJ that can quickly and reliably switch with acceptable thermal stability using a reduced programming current, and that is suitable for high volume manufacture and widespread durable use in reduced-scale data storage devices.